1. Field of the Invention
The present invention relates to an optical writing device, an image forming apparatus, a method and program for controlling the optical writing device, and a recording medium, and more particularly, to skew correction of an image while suppressing degradation of the image quality.
2. Description of the Related Art
Recently, digitization of information tends to be promoted, and image forming apparatuses, like printers and facsimile machines used for output of digitized information and scanners used for digitization of documents, have become essential apparatuses. Many image forming apparatuses include imaging functions, image forming functions, communication functions, and the like, to be configured as multifunction peripherals that are usable as printers, facsimile machines, scanners, and copiers.
Of these image forming apparatuses, electrophotographic image forming apparatuses are widely used as image forming apparatuses used for output of digitized documents. An electrophotographic image forming apparatus forms an electrostatic latent image on a photosensitive element by exposing a photosensitive element, and develops the electrostatic latent image into a toner image using a developer such as toner, and transfers the toner image onto a sheet of paper to output the sheet of paper.
There are several types of optical writing device which exposes a photoreceptor to an optical beam in an electrophotographic image forming apparatus: a laser diode (LD) raster optical system type and a light emitting diode (LED) writing type. The LD raster optical system type of optical writing device includes a light source, which emits a beam to expose a photoreceptor, and a deflector, such as a polygon scanner, for deflecting the emitted beam to scan the entire surface of the photoreceptor; the LED writing type of optical writing device includes an LED array (LEDA) head. In such an optical writing device, the LD raster optical system type has an error due to skew, positional shift, etc. of an f-theta lens and a reflecting mirror; the LED writing type has skew, an installation error, etc. of the LEDA head.
One of problems caused by such an error is skew of a scanning beam, i.e., a problem that a main scanning direction of an optical beam is inclined from an original direction. As a method to correct this skew, there are several methods applicable to both the LD type and the LED type; one of the methods is to mechanically correct the skew, and another method is to deform an image to be output by image processing according to an amount of the skew thereby finally forming a suitable image.
In the method to mechanically correct the skew, correction is achieved by providing an adjustment mechanism which displaces the mounting positions of a mirror and a head in a writing unit in the case of the LD type and the mounting position of the LEDA in the case of the LED type; however, to automatically carry out this adjustment, an actuator, such as a motor, is required, and this results in an increase in cost of the entire device.
On the other hand, in correction by image processing, the skew is corrected in such a manner that pixels composing an image are shifted in a sub-scanning direction at a certain main-scanning direction position. As a method to shift pixels composing an image in the sub-scanning direction, there are several methods: a method to accumulate pixels of respective main scanning lines in line memories with respect to each main scanning line and switch the line memory from which pixels are to be read out according to a main-scanning direction write position thereby shifting the image in the sub-scanning direction and a method to store pixels in a line memory in a state where the pixels are shifted in the sub-scanning direction at a certain main-scanning direction position. In this case, we only have to add a line memory to an image processing unit in accordance with a correction range, so this can be achieved at relatively low cost as compared with the mechanical correction, and also this allows automatic correction, and therefore, this is effective as a method of skew correction.
However, in the method to correct skew by image processing, a change in dither pattern arises at the image shift position. The change in dither pattern changes a relationship of adjacent pixels in the main scanning direction, such as a change from a white pixel to a black pixel, and an area of attachment of toner at the time of output varies. In an image represented by pseudo gradation processing such as a dither method, this variation in area of attachment of toner frequently occurs successively or periodically in the sub-scanning direction and appears as a streaky noise extending in the sub-scanning direction on an image formed on a print sheet.
To cope with such a problem, there is proposed a method to prevent appearance of a streaky noise described above by performing image shift at the position where no dither matrix image pattern exists (for example, see Japanese Patent Application Laid-open No. 2009-27683). Furthermore, there is proposed a method to store information on skew in a storage unit and control the timing to drive each element on the basis of the information (for example, see Japanese Patent Application Laid-open No. H11-70697).
In a case of using the technology disclosed in Japanese Patent Application Laid-open No. 2009-27683, image shift is performed while avoiding an area of dither matrix; therefore, the image shift period has to be at least longer than the area of dither matrix, and the number of correctable lines is limited. Furthermore, this technology cannot be applied to such an image that a dither pattern is expanded over the whole page.
In a case of using the technology disclosed in Japanese Patent Application Laid-open No. H11-70697, a configuration of a circuit for controlling the timing to emit a light with respect to each element is complex, resulting in increases in the circuit size and cost.
Furthermore, in a conventional skew correction method, it is a popular way that a controller of an image forming apparatus performs skew correction and then inputs image data subjected to the skew correction to an optical writing device. In this case, there is individual variability in skew among optical writing devices, so the optical writing device has to input skew information to the controller, and this constrains the design of the optical writing device and the controller.
Moreover, it is uncertain what kind of skew information is input from the optical writing device; therefore, at the time of design of a skew correction function of the controller, it is necessary to design the skew correction function so as to deal with any kind of skew or design the skew correction function to meet skew which has occurred in the optical writing device, and this results in constraints on the design and an increase in cost. Such problems can be resolved by providing a skew correction function to the optical writing device.